Lithography is a mechanism by which a pattern on a mask is projected onto a substrate such as a semiconductor wafer. In areas such as semiconductor photolithography, it has become necessary to create images on the semiconductor wafer which incorporate minimum feature sizes under a resolution limit or critical dimension (CD). Currently, CDs are reaching 65 nanometers and less.
Semiconductor photolithography typically includes the steps of applying a coating of photoresist on a top surface (e.g., a thin film stack) of a semiconductor wafer and exposing the photoresist to a pattern. A post-exposure bake is often performed to allow the exposed photoresist, often a polymer-based substance, to cleave. The cleaved polymer photoresist is then transferred to a developing chamber to remove the exposed polymer, which is soluble to an aqueous developer solution. As a result, a patterned layer of photoresist exists on the top surface of the wafer.
Immersion lithography is a new advance in photolithography, in which the exposure procedure is performed with a liquid filling the space between the surface of the wafer and the lens. Using immersion photolithography, higher numerical apertures can be built than when using lenses in air, resulting in improved resolution. Further, immersion provides enhanced depth-of-focus (DOF) for printing ever smaller features.
The immersion exposure step may use de-ionized water or another suitable immersion exposure fluid in the space between the wafer and the lens. Though the exposure time is short, the fluid can cause heretofore unforeseen problems. For example, droplets from the fluid can remain after the process and can adversely affect the patterning, critical dimensions, and other aspects of the resist.
Efforts have been made to reduce the occurrence of droplets on the wafer. One example is to provide a drying process immediately after the exposure, such as is described in the presently incorporated Provisional Application No. 60/695,562 filed Jun. 30, 2005. However, the drying process must occur very quickly, e.g., within a few minutes, to prevent some types of damage. Sometimes it is difficult to ensure that the drying process is done in a sufficiently quick amount of time.